Residents across Barry, Penarth, Llantwit Major and Cowbridge are being urged to prepare for possible disruption.
The warning is in force from 11am until 11.59pm on Sunday, August 30. Forecasters say heavy showers, lightning and strong winds could affect parts of south Wales throughout the day.
The Met Office expects thunderstorms to move into Wales later on Sunday morning before becoming more widespread during the afternoon.
Some areas could receive 10mm to 15mm of rain in less than an hour, while isolated locations may get higher totals over several hours.
Heavy downpours could create difficult driving conditions on roads around Barry, Penarth, Dinas Powys and the wider Cardiff commuter belt.
Spray, standing water and reduced visibility may cause delays for motorists. Bus and rail services could also be affected as storms develop, with the Met Office warning of possible train delays.
Lightning strikes could damage a small number of buildings and other structures. Short-term power cuts and interruptions to other services are also possible.
Localised flooding is another concern, especially in places where surface water builds up during intense rainfall.
However, not every community will experience the worst conditions. Thunderstorms can be highly localised, so severe downpours may affect one place while nearby areas remain largely unaffected.
People with outdoor plans, including visits to beaches, parks and attractions across the Vale, are being advised to monitor forecasts closely.
Anyone caught outside during a thunderstorm should seek shelter inside a substantial building or vehicle. People should avoid taking cover beneath trees, which can be vulnerable to lightning strikes.
Residents are encouraged to charge mobile phones and keep torches available in case of a power outage.
It says risks include heavy rain and lightning, but disruption is not expected everywhere.
Those planning outdoor activities should monitor weather forecasts closely throughout the day.
The warning expires at 11.59pm, with conditions expected to improve later in the evening as the heaviest showers become more isolated.
The Met Office has given the warning a medium likelihood of impacts but relatively low overall severity.
